1. Field of the Invention
The present invention relates to oceanographic measurement systems, and more particularly to a three-axis current meter for measuring all three components of current velocity in the absence of self-interference caused by the wake of the sensor supports.
2. Description of the Prior Art
Current meters have been developed to measure the differential travel time of acoustic signals traveling with and against the fluid flow. This concept has been implemented by (1) short pulse using two transmitters and two receivers for each axis, (2) short pulse using two transducers each acting as both a transmitter and receiver, (3) dual "sing-around" sound velocimeters with straight line sound paths in opposite directions, (4) continuous wave using two widely different high frequency carriers, and (5) continuous wave bursts using a single frequency on a single pair of transducers. The first three methods require the measurement of arrival time differences of pulses with sufficient speed to resolve currents less than 1 cm/sec and the fourth method requires the measurement of phase differences to very short time resolutions which require extremely high speed circuitry and auto calibration features.
The fifth method allows a slower time measurement which drastically simplifies the circuitry and eliminates the requirement for ultra high speed circuitry and for auto calibration. A current configuration provides two vertically separated horizontal acoustic paths which intersect at 120.degree. and a third vertical path through the intersection point. This spatial arrangement ensures that no wake of any transducer crosses any of the acoustic paths, but there is still some self-interference present which limits the sensitivity of the meter.